Plastic action fatigue resistant nut



May 28, 1968 N. c. DAHL, 3,385,339

PLASTIC ACTION FATIGUE RESISTANT NUT Filed July 7, 1967 5 Sheets-Sheet 1F i .I

7 ad /i Z/ 22- a 7 May 28, '1 968' c. DAHL 3,385,339

PLASTIC ACTION FATIGUE RESISTANT NUT Filed July v, 1967 5 Sheets-Sheet 2l 1 i I I 4 (was F ID I a; arm/ m Will/6777011! May 28, 1968 N. c. DAHL3,385,339

PLASTIC ACTION FATIGUE RESISTANT NUT Filed July 7, 1967 3 Sheets-Sheet 3v T O V BOET STRESS BOLT STRESS TOTAL BOLT m X DUE TQ AXIAL DUE TO LOADSTRESS FORCES [a BOLT FORCE TRANSFER (T l BOLT STRESS BOLT STRESS TOTALBOLT max R, DUE o AXIAL. DUE TO LOAO' STRESS 5 BOLT FORCE TRANSFERFORcEs A ,A L

Tmux BOLT sTREss BOLT STRESS TOTAL BOLT F; DUE TO AXlAL DUE TO LOADSTRESS BOLT FORCE TRANSFER 1 l4 FORCES United States Patent 3,385,339PLASTIC ACTION FATIGUE RESISTANT NUT Norman Christian Dahl, 40 Fern St,Lexington, Mass. 02173 Continuation-impart of application Ser. No.501,902, Oct. 22, 1965. This application July 7, 1967, Ser. No. 659,831

Claims. (Cl. 151-21) ABSTRACT OF THE DISCLOSURE A nut having twoadjacent, axially spaced, internally threaded sections which areconnected by an integrally formed web. One threaded section is displacedapart and out of phase with the other section. When the nut is engagedwith a bolt, the leading faces of the first section of the nut engagethe lagging faces of the bolt threads, while the lagging faces of thesecond section engage the leading faces of the bolt threads. When thenut is loaded, the web plastically deforms and the thread engagement ofthe first section with the bolt is the same as the second sectionthereby redistributing the load transfer intensity Cross reference torelated application This is a continuation-in-part of my copendingapplication, Ser. No. 501,902, filed Oct. 22, 1965, now abandoned.

This invention relates to fatigue resistant nuts.

It has been found that the ordinary nut and bolt unit will frequentlyfail due to metal fatigue of the bolt. Most often the failure of thebolt will occur at the bolt threads which are at the end of the nutclosest to the application of the load. The type of failure hererelevant is that due to repeated stressings over a period of time, not asingle overloading. When the load is transferred from a bolt to a nut inthe usual way (bolt in tension, nut in compression) the transfer occursin a non-uniform manner along the thread engagement between the nut andbolt. The load transfer will be maximum at the first engaged thread inthe direction of the load and decrease, but not linearly, to a minimumat the last engaged thread.

It is a primary object of the invention to provide a fatigue resistantnut which will substantially improve the fatigue life of the nut-boltcombination in which it is used.

It is a further object of the invention to provide a fatigue resistantnut which will effect a close approximation to the ideal load transferintensity distribution between the threads of the nut and an associatedbolt, 2. distribution which will make fatigue failure of the boltequally likely to occur at all points along the length of the boltthread in engagement with the nut and thus use the potential strength ofthe nut-bolt combination in the most efficient manner.

A further object of the invention is to provide a fatigue resistant nutin which the load transfer intensity distribution is substantiallyreversed from that which occurs in an ordinary nut and bolt relation.

A further object of the invention is to provide a fatigue resistant nutwhich will conform itself to meet a particular set of conditions in itsinitial loading cycle in the plastic region and thereafter respondelastically to similar loads.

A further object of the invention is to provide a fatigue resistant nutwhich is simple in design and inexpensive to manufacture.

The invention features a nut comprising at least one pair of adjacent,\axially spaced, internally threaded sections connected by a web, thethreads of one of said sections being displaced apart and out of phasewith the threads of the other by more than either the threads clearance(backlash) or the yield displacement of the web but less than the threadclearance plus one-half of the thread pitch, said sections and web beingintegrally formed, said web being designed to deform plastically toadjust to said bolt and load conditions during its initial loadingcycle, thereafter responding elastically to cycles of similar loadconditions.

Other objects, features and advantages will appear from the followingdescription of a preferred embodiment of the invention, taken with theattached drawings in which:

FIG. 1 is partial sectional view of one embodiment of the inventionshowing a nut of the invention threaded on a bolt but not tightened;

FIG. 2 is a sectional view of a portion of the threads shown in FIG. 1under initial tightening of the nut on the bolt;

FIG. 3 is a sectional view of a portion of the threads shown in FIG. 1under further (transitional) tightening of the nut;

FIG. 4 is a sectional view of a portion of the threads shown in FIG. 1under final tightening of the nut and under subsequent load conditions;

FIG. 5 is an enlarged sectional view of the bolt threads shown in FIG.1;

FIG. 6 is a sectional view of another embodiment of the invention;

FIG. 7 is a sectional view of a third embodiment of the invention;

FIG. 8 is a graph representing the load transfer intensity distributionin an ordinary nut and bolt unit;

FIG. 9 is a similar representation of the ideal load transfer intensitydistribution in a nut and bolt unit in accordance with the invention;

FIG. 10 shows the correlation between means stress and alternatingstress and fatigue life for common materials from which bolts aremanufactured;

FIG. 11 is a graph illustrating the elastic and plastic behavior of theweb in the nut of the invention;

FIG. 12 is a diagram illustrating stress distribution in a bolt using anut of the invention;

FIG. 13 is a diagram similar to that of FIG. 12, but illustrating boltstress when a normal nut is used; and

FIG. 14 is a diagram similar to those of FIGS. 12 and 13, illustratingthe bolt stress when a nut is used that has its threads displaced out ofphase toward each other.

Referring to the drawings, there is shown in FIG. 8 an indication of theload transfer intensity distribution between the threads of an ordinarynut and bolt combination. The load transfer intensity between eachthread of the nut and its corresponding bolt thread decreasesnon-linearly as the distance of the thread from the direction of theload increases. Thus, the nth thread (in FIG. 5) will transfer less loadthan the first.

If a metal is stressed (stress being the load per unit area) repeatedlywith various combinations of mean stress a and alternating stress er themaximum stress being (o -Fa and the minimum stress being (a a' thecombinations of these quantities rendering equal fatigue life will be asshown in FIG. 10. Alternating stress is plottedalong the ordinate andmean stress along the abscissa. The curves C C C and C represent thecombinations of means stress and alternating stress which give equalfatigue life (equal number of cycles of alternating stress beforefailure), where C is the lowest number of cycles and C is the highest.These curves are constructed by applying a fixed mean stress to aspecimen and then repeatedly subjecting it to a given alternating stressuntil it fractures, and then repeating the test for differentcombinations of stress.

A better understanding of the problem may be had by subjecting a nut andbolt combination to a varying loading condition such that the load inthe bolt is:

P=P iP P=P (l:e) (II) where P =mean load P =alternating load (III) Withthe nut and bolt combination behaving elastically the intensity of theload transfer at the kth thread in FIG. 5 will be k= km ka km( i where F=mean load transfer intensity at bolt thread k F =alternating loadtransfer intensity at bolt thread k and the tensile force in the bolt atthe kth thread P will be P 1 km ka =P (lie) (V where P ==mean tensileforce at bolt thread k P =alternating tensile force at bolt thread k(VII) In general, the effective fatigue stress at the root of the kththread of the bolt is represented by the sum of the stress due to thetensile force and the stress due to the load transfer intensity asfollows:

where a, [3 and 7 include the effect of stress concentrations. The thirdquantity in (VIII) is absent for the thread engaged closest to thedirection of the force, thread 1 in FIG. 5, thus,

Combining the expressions of (IV) and (VI) the mean stress andalternating stress can be expressed in the form It is apparent from (X)that all the alternating and mean stress components will be in the ratioof e, represented by points along line OF of FIG. 10.

Since all values of 17,, and a must be along line OF of FIG. 10,calculations of and o' combinations for threads 1, 2, 3 n of FIG. willalso be on line OF. When such calculations are made it will be notedthat point 1, corresponding to first engaged thread 1 of FIG. 5, showsfracture at the low end of the life cycle curves while the point 11,corresponding to the nth thread in FIG. 5, indicates fracture at thehigh end of the life cycle curves. This correlates well with thefindings that in normal nut and bolt combinations under such conditionsfracture occurs most commonly in the bolt at its first engaged thread.

A nut configuration of optimum fatigue resistant characteristics wouldbe one that makes failure of the bolt due to fatigue equally likely atall points along the engagement of the thread or, stated conversely,equally unlikely at any given thread. In a normal nut and boltcombination the tensile force P in the bolt at thread k decreases fromthe first engaged thread 1 of FIG. 5 to the last or nth thread. Thestress due to this force will likewise decrease as the nth thread isapproached. The load transfer intensity F represents the proportion ofthe bolt load P which is transferred to the nut at the kth thread, andin a normal nut and bolt combination this, too, decreases from the firstto the nth thread. If however, the load transfer intensity can be madeto vary inversely to P as in FIG. 9, it would be possible for theincreasing load transfer intensity to compensate for the decreasingtensile force P so that all threads would have total i stresses of equalmagnitude. Thus, all the threads 1, 2, 3 n would be located on the samelife cycle curve and have equal fatigue life.

An approximation to such an inverse load transfer intensity is producedby the configuration incorporated in the nut 10, FIG. 1 having twoadjacent, axially spaced, internally threaded sections 16 and 18 andconnecting web 20. Web 20 is set off from the outer periphery of the nutby recess 22 and from its inner periphery by groove 24. Both recessesand grooves may be employed as shown in FIGS. 1 and 7 or they may bestaggered as shown in FIG. 6; they may be employed separately usingeither a recess or a groove.

The first engaged or lower section 18 contains internal threads 26 whichduring manufacture are displaced apart and out of phase with internalthreads 28 of upper section 16. With nut 10 secured lightly againstplate 14 on bolt 12, bolt thread 30 will have its leading faces 32engaging the lagging faces 34 of upper section threads 28 in the normalmanner of engagement, while its lagging faces 38 will engage leadingfaces 36 of lower section threads 26, in reverse of the normal manner ofengagement.

One manner of displacing threads 28 apart and out of phase with respectto threads 26 is to rotate either section 16 or 18 relative to the otherthrough an angle beyond the elastic limit of the web material so thatwhen the rotating force is released, sections 16 and 18 Will remainpermanently rotated and out of phase. This results in an effective axialdisplacement apart of threads 26 and 28 without relative axial movementof sections 16 and 18. Displacement of threads 28 apart and out of phasewith respect to threads 26 can be accomplished in other ways such as byactually displacing sections 16 and 18 apart permanently in an axialdirection relative to each other.

If the nut threads are displaced out of phase by permanently rotatingthe sections 1.6 and 18 relative to each other the outer surface of theupper section 16 could be made cylindrical in shape with a diametersufiiciently small so that it would not interfere with a wrench or otherturning means used to tighten the nut by engaging the outer surface ofthe lower section 18. Avoidance of interference in the wrenching processdue to rotational misalignment of the two sections of the nut may alsobe accomplished by twisting the sections, then threading them andtwisting them back to their original relation, leaving the threadsdisplaced but the outer portions of the nut aligned.

The threads 28 must be displaced apart and out of phase with threads 26by a distance which is greater than either the axial thread clearance,the axial distance left between mating threads to allow ease ofassembly, or the yield displacement ;y of the Web shown in FIG. 11 butless than the axial thread clearance plus one half the thread pitch thethread clearance is shown in FIG. 1 as the space 40 between threads 30and 28. If the threads phasing is displaced by this distance, uppersection nut threads 28 will engage bolt threads 30 on the opposite facesfrom lower section nut thread 26 in the manner shown in FIG. 1. When anut is designed without a groove 24, the threads of its adjacentsections will be continuous and the correct phase displacement willoccur gradually across the portion of the threads between the sectionsseparated by the recess 22.

In displacing the threads of adjacent sections apart and out of phasewith each other, the actual separation between the sections can amountto many times the pitch without adversely affecting the desired threadrelation, so long as the threads are finally out of phase by more thaneither the thread clearance or the yield displacement of the web andless than the thread clearance plus one half of the thread pitch.

In FIG. 2, where load P, less than L is acting on bolt 12 andcorresponding compressive load P is exerted on nut 10, the threads 26,28 and 30 are in the same relation as in FIG. 1, described above, sincethe web 20 is loaded in the elastic region. Web loads within the elasticregion (the regiOn including the loads to which a material may besubjected and still be able to return to its original form upon removalof the load) are on the steep portion of the curve in FIG. 11, where asubstantial increase in load L applied to the web will cause only aslight decrease in the length of the web. When the load L on the web 20reaches a value L the yield point (the point at which deformation firstincreases markedly with little increase in applied load), there can be asubstantial decrease in the length of web 20 with only a slight increasein the load L. When P is increased beyond L the contact pressure betweenthre lower section nut thread leading faces 36 and bolt thread laggingfaces 38 will decrease and when P is sufficiently larger than L the web20 will decrease in length enough by plastic (permanent) deformation sothat the thread faces 36 and 38 will disengage and there will exist thetransitional loading condition shown in FIG. 3 where only the uppersection nut threads 28 are in contact with the bolt threads 30. As thebolt load P is increased further there is further plastic deformation ofthe web 20 until the end of the transitional region is reached at pointa in FIG. 11 at which point the load P=P =L is such that the lowersection nut thread lagging faces 38 have come in contact with the boltthread leading faces 32 and the relationship of the threads shown inFIG. 4 prevails; at this point the lower section nut threads 26 transfersubstantially none of the bolt load.

As the load P on the bolt is increased beyond L the additional load iscarried primarily by the nut threads 26 in the lower section 18. Thiscondition results from the fact that any substantial increase in theload carried by the nut threads 28 in the upper section 16 and thus bythe web 20, would require a large increase in the compression of the web20 and this compression could not take place unless the nut threads 26in the lower section 18 or the mating bolt threads deformed plasticallyor sheared due to transfer of load. Assuming the threads are properlydesigned so that such plastic deformation or shearing of threads doesnot take place, it is clear that most of the load beyond L is taken upby the threads 26 of the lower section 18 of the nut. Thus, when thereis a load P=P acting on the bolt, where P is greater than L,,, the web20 will be in the condition indicated by the point b in FIG. 11 wherethe load L carried by the web 20 is less than P by the amount of loadcarried by the lower section threads 26.

If now the load on the bolt is reduced from P to zero, during thisunloading process the web 20 will move from the condition 12 to thecondition 0 in FIG. 11. The path be will be an elastic unloading. It nowthe load on the bolt is again raised to its formed value P the web 20moves along the path ab and reaches the condition b where the threads inthe upper portion 16 once again carry the same load L, as was carriedpreviously. The path cb is an elastic loading path. All subsequentunloading and loading of the bolt, typical of fatigue loading, will beaccomplished with elastic action of the web 20 so long as the maximumload on the bolt does not exceed the value P If load P later exceeds itsprevious maximum P a slight plastic deformation of web 20 similar tothat occurring during the original loading from point a to point b wouldtake place during this increase in bolt load. Web 20 will now behaveelastically under all future bolt unloadings and loadings which do notexceed this new maximum value of P.

The area of the web 20 is made such that when the maximum load P isacting on the bolt the load L carried by the web is such that the stressin the bolt due to both axial tensile force and load transfer intensityhas the same maximum value in the first bolt thread engaged by the lowernut section 18 and the first bolt thread engaged by the upper nutsection 16. This design condition is illustrated in FIG. 12. Forcomparison there is illustrated in FIG. 13 the stress distribution in abolt engaged by an ordinary nut and carrying the same load as the boltin FIG. 12.

Because the threads of the two nut sections are spaced apart out ofphase a distance greater than either the maximum axial thread clearanceto be encountered in practice or the yield displacement of the web 20,the exact tolerance with which either the bolt or nut is made is notimportant since when the thread relationship shown in FIG. 4 is reachedit is certain that the web has undergone plastic deformation and theload in the web is close to the design load L Thus, because of thedesign dependence upon plastic deformation of the web the nutautomatically fits itself to each bolt to produce a stress dis tributionin the bolt which will vary from the ideal shown in FIG. 12 only by asmall amount for rather large variations in the tolerance with which thebolts or nuts are made. In contrast, if the nut sections were displacedapart only enough so that the web was intended to act elastically thensmall differences in tolerances would have substantial differences onthe load carried by the web and hence, on the stress distribution in thebolt.

It is of importance to note that if the two nut sections were spacedtogether out of phase, e.g. if the web 20 were compressed permanentlyafter the threads had been formed in the two sections of the nut, thenthe maximum stress in a bolt supported by such a nut would be greaterthan the maximum stress in a bolt supported by an ordinary nut. This isshown in FIG. 14 which illustrates the case where a portion of the boltwithin the nut continues to experience an axial compressive force evenwhen the bolt is loaded with a tensile force.

Other nut designs which will deliver a desirable load transferdistribution are shown in FIGS. 6 and 7. Nut 42 of FIG. 6 contains fouradjacent, axially spaced internally threaded sections 44, 46, 48 and 50connected by webs 52, 54 and 56 which decreases in thickness from bottomto top, having internal grooves 58, 60 and 62, and external recesses 64and 66, and which deform in shear parallel to the axis of the nut in themanner illustrated in FIG. 11. In this construction the threads in eachof the sections 44, 46, 48 and 50 are displaced apart and out of phasewith the threads of the adjacent sections by an amount which is morethan either the axial thread clearance or the yield displacement of theintervening web and less than the axial thread clearance plus one halfof the thread pitch. The nut 68 of FIG. 7 is similar to that of FIG. 6except that the internal grooves 70, 72 and 74 are in line with externalrecesses 76, 78 and 80. Webs 86, 84 and 82 decrease in thickness, web 86being thicker than web 84 and web 84 being thicker than web 82. Thisvariation in web thickness in the nuts of FIGS. 6 and 7 produces acloser approximation to the ideal load transfer distribution and insuresthat the webs which are screwed on a bolt after a preceding web or webswill having successively lower yield points and will not force plasticdeformation of those preceding webs.

Other embodiments will occur to those skilled in the art and are withinthe following claims.

What is claimed is:

'1. A fatigue resistant nut for engagement with a threaded membercomprising:

at least two adjacent, axially spaced, internally threaded sections ofsubstantially identical thread formation connected by an integrallyformed web;

said web defining an annular portion having an inner diameter at leastas great as the root diameter of the thread with the radial thickness ofsaid web being less than the radial thickness of the threaded sectionsfrom the roots of the threads to the external surfaces of said sections;

the area of said web being such that the area times the yield stresswill be such that the web will deform plastically when said nut isthreaded on said memher and said member is subjected to load; and

the threads of one of said sections being displaced apart from and outof phase with the threads of the adjacent said section by more thaneither the axial thread clearance or the yield displacement of said Webbut less than said axial thread clearance plus one-half of the threadpitch.

2. The nut of claim 1 wherein the internally threaded sections include afirst and second section.

3. The nut of claim 2 wherein:

the first engaged sections threads are in reverse load transferringrelation with the threads of said member and the second sections threadsare in normal load transferring relation with said members threads whenthe load on said member is less than the elastic limit load of said web;said first engaged sections threads changing from a reverse loadtransferring relation to a normal load transferring relation with saidmembers threads as the load on said member is increased through thetransitional region of the plastic region of said web and said webundergoes plastic deformation; and said first engaged sections threadsand said second sections threads are in normal load transferringrelation with said memberss threads as the load on said member isincreased to a predetermined maximum load beyond said transitionalregion of said plastic region of said web;

said first engaged sections threads and said second sections threadsremaining in normal load transferring relation with said members threadsthereafter, and said web responding elastically to repeated unloadingand reloading of said member not exceeding said previous maximumloading.

4. The nut of claim 2 in which:

the leading faces of said first engaged sections threads initiallyengage the lagging faces of said members threads and the lagging facesof said second sections threads initially engage the leading faces ofsaid members threads; said web being plastically deformable so that assaid member is loaded the lagging faces of said first engaged sectionsthreads also engage the leading faces of said members threads; saidlagging faces of said first engaged sections threads and of said secondsections threads engaging said leading faces of said members threadsthereafter, the total stress in said member being substantially equal atthe first thread engaged by each of said threaded sections, and said webresponding elastically to subsequent unloadings and reloadings of saidmember.

5. The nut of claim 1 wherein the threaded sections comprise a pluralityof threaded sections, each pair connected by an integrally formed web,each of said sections threads being displaced apart and out of phasewith the threads of its adjacent sections by more than either the axialthread clearance or the yield displacement of said web and less than thesaid axial thread clearance plus one half of the thread pitch.

6. The nut of claim 5 in which:

under initial loading with said member said first engaged sectionsthreads are in reverse load transferring relation with the threads ofsaid member and said other sections threads are in normal loadtransferring relation with said members threads, under loads less thanthe elastic limit of the first web connecting said first engaged sectionwith the next adjacent section; said first engaged sections threadschange from a reverse load transferring relation to an intermediate loadtransferring relation with said members threads as the load on saidmember is increased through the transitional region of the plasticregion of said first web and said first Web undergoes plasticdeformation; and said first engaged sections threads and said othersections threads are in normal load transfer-ring relation with saidmembers threads as the load on said member is increased to apredetermined maximum load beyond said transitional region of saidplastic region of said first web;

said first engaged sections threads and said other sections threadsremaining in normal load transferring relation with said members threadsthereafter, and said first web responding elastically to repeatedunloading and reloading of said member not exceeding said previousmaximum loading.

7. The nut of claim 5 in which:

the leading faces of said first engaged sections threads initiallyengage the lagging faces of said members threads and the lagging face ofsaid other sections threads initially engage the leading faces of saidmembers threads; said first web being plastically deformable so that assaid member is loaded the lagging faces of said first engaged sectionsthreads also engage the leading faces of said members threads; saidlagging faces of said first engaged sections threads and of said othersections threads engaging said leading faces of said members threadsthereafter, the total stress in said member being substantially equal atthe first thread engaged by each of said threaded sections, and saidfirst web responding elastically to subsequent unloadings andre'loadings of said member.

8. The nut of claim 5 wherein each succeeding web is designed to yieldplastically in succession as the load on said member is increased to apredetermined maximum load.

'9. The not of claim 8 which includes four threaded sections and first,second and third webs joining said sections, the third web designed toyield plastically prior to the second web yielding and the second webdesigned to yield plastically prior to the first web yielding and thefirst web yields plastically in succession as the load on said member isincreased to a predetermined maximum load.

10. The nut of claim 1 in which the threads of adjacent sections aredisplaced apart and out of phase by exceeding the elastic limit of thematerial of the nut.

References Cited FOREIGN PATENTS 853,081 10/1952 Germany.

CARL W. TOMLIN, Primary Examiner.

R. S. BRITTS, Assistant Examiner.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No.3,385,339 May 28, 1968 Norman Christian Dahl It is certified that errorappears in the above identified patent and that said Letters Patent arehereby corrected as shown below:

In the drawings, Fig. l, the pitch of the bolt threads should beindicated by reference lines extending from a pair of adjacent threadcrests having a double-headed arrow therebetween, and the pitch shouldbe identified by reference numeral 90 having a lead line extending tothe arrow. Fig. 10, "c should read c "c should read c "c should read cand a parenthesis should appear after "stress". Fig. 11, "transitionalarea" should be transitional region 'L should read L "'L should read L adotted line should extend perpendicularly upwardly from the Compressionof Web axis to the end of the dotted line representing the yield pointload L and 0y should identify the point at which the perpendiculardotted line intersects the compression of Web axis. A bracket shouldalso appear between points "b" and "c" on the graph along with thestatement elastic behavior upon unloading from b and reloading to bSigned and sealed this 13th day of January 1970.

(SEAL) Attest:

EDWARD M.FLETCHER,JR. WILLIAM E. SCHUYLER, JR. Attesting OfficerCommissioner of Patents

